The invention relates to a disk apparatus, preferably a hard disk apparatus, comprising a disk provided with a plurality of tracks running parallel to each other on the disk, a track comprising servo fields alternating with data fields, a servo field comprising an AGC field having an AGC signal recorded in it and a Gray code field having an encoded Gray bit sequence recorded in it, a data field for recording a data information signal in it.
Such disk apparatus is known from U.S. Pat. No. 5,661,760, document D1 in the list of related documents that can be found below. Generally, a hard disk is subdivided into a plurality of circular zones. FIG. 20 shows schematically a portion of such a hard disk. In each zone, such as the zones z1, z2, z3 in FIG. 20, data is written/read in circular tracks with a bitrate which is substantially constant, but increases for further outwardly lying zones on the hard disk. This results in a substantially constant data density on the hard disk, because of the fact that the rotational velocity vd of the hard disk is constant. Each track comprises servo fields alternating with data fields.
The servo fields form so-called servo spokes on the hard disk. Four of such spokes s1 to s4 are shown in FIG. 20. They are in the form of prerecorded servo signals that serve for positioning a read/write head on the hard disk. The servo signals can be recorded on the hard disk in two ways. In one way, the servo signals are written with a constant frequency, irrespective of the radial position on the hard disk. This results in spokes whose width increases for positions lying further outwardly on the hard disk. In such layout, the frequency of the signals in the servo fields thus have no relation with the bitrate of the data written on the hard disk. In a second way, the frequency of the signals in the servo fields have a relation with the bitrate of the data written in the zones. This results, upon average, in shorter servo fields than the servo fields in accordance with the first way described above. FIG. 20 shows the servo fields recorded in accordance with the second way by means of the black areas in the various zones. Further, when recorded in accordance with the first way, the servo fields occupy a portion on the disk indicated by the black portion and the hatched portion, as shown in the spoke s1.
The invention aims at providing an improved format for the signals in the servo fields on a disk, preferably a hard disk. The invention is applicable in any of the two formats described above. In accordance with the invention, the disk apparatus as defined in the opening paragraph is characterized in that the AGC signal has a frequency fAGC and the Gray bit sequence has a bitfrequency fGray, which frequencies satisfy the following relationship:
fGray/fAGC=n,
where n is an integer for which holds nxe2x89xa71,
and that the Gray bit sequence has been encoded in accordance with a p-phase modulation code, where p is an integer for which holds p greater than 1.
The invention is based on the following recognition. By using a p-phase modulation code for the Gray bit sequence stored in the Gray code field and preferably also for bit sequences stored in other fields, such as the synchronization field and the auxiliary field (in some publications also named as RROC field), the signal-to-noise ratio is increased, eg. compared to dibit signalling. Further, in combination with the requirement that fGray/fAGC=n, where n is an integer for which holds nxe2x89xa71, clock extraction is improved and becomes possible throughout these fields. As a result, the AGC field can be shorter and the so-called ZPR (=zero phase restart) field can be dispensed with. Equivalent relationships exist for the ratios fsync/fAGC and fAUX/fAGC, in accordance with the claims 2 and 3, respectively. The ratio n need not necessarily be the same for fGray, fsync and fAUX, but preferably they are.
For bi-phase (p=2), preferred values for n are 1 and 2. The choice for the value of n depends on whether a higher density is aimed at, or whether an increased signal to noise ratio is the objective. Higher values for n result in a higher density, but in a decrease in signal to noise ratio. For quad phase (p=4), the preferred value for n is 2. For higher values of n the signal to noise ratio decreases to a less desirable value. Modulation codes for which p=3 are particularly applicable to optical disks.